1. Field of the Invention
The present invention relates generally to wireless communication systems, and, more particularly, to methods and devices for transmitting and receiving synchronous channels and broadcasting channels.
2. Description of the Related Art
In 3GPP Long Term Evolution (LTE) systems, each radio frame has a length of 10 ms, which is divided into 10 sub-frames. One downlink transmission time interval (TTI) is defined on one sub-frame. FIG. 1 shows the frame structure of the Frequency Division Duplex (FDD) system, where each downlink sub-frame consists of two slots, each slot comprises 7 Orthogonal Frequency Division Multiplexing (OFDM) symbols in length for a normal cyclic prefix (CP), and each slot comprises 6 OFDM symbols in length for an extended CP. FIG. 2 shows the frame structure of the Time Division Duplex (TDD) system, where each radio frame is divided into two half frames, each 5 ms long, and sub-frame 1 and sub-frame 6 each comprise 3 special domains, namely, a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP) and an Uplink Pilot Time Slot (UpPTS), for an overall length of 1 ms.
FIG. 3 shows the sub-frame structure of an LTE system. The first n OFDM symbols, where n equals 1, 2 or 3, are for transferring downlink control information including Physical Downlink Control Channel (PDCCH) and other control information, and the remaining OFDM symbols are used for transferring the Physical Downlink Shared Channel (PDSCH). Resources are allocated per user in the time domain and frequency domain. A Physical Resource Block (PRB) is the basic unit of allocation, comprising 12 consecutive sub-carriers in frequency, corresponding to one slot in the time domain. If the sub-frame consists of two consecutive slots, two PRBs within two slots on the same sub-carrier within one sub-frame are named a PRB pair. Each Resource Element (RE) within each PRB pair, is the smallest unit of time-frequency resource, i.e. it is a sub-carrier in frequency, but an OFDM symbol in time. REs can play different roles, for example, some REs are used for transmitting a Cell-specific Reference Signal (CRS), some REs are used for transmitting a User-specific Demodulation Reference Signal (DMRS), some REs are used for transmitting a Channel State Indication Reference Signal (CSI-RS), etc.
In an LTE system, the Synchronous channel (SCH) is transmitted with a periodicity of 5 ms. As shown by FIG. 4, in an FDD radio frame, a Primary Synchronous channel (P-SCH) is located in 72 sub-carriers in the middle of the last OFDM symbol of slot 0 and slot 10, and a Secondary Synchronous channel (S-SCH) is located in 72 sub-carriers in the middle of the second-to-last OFDM symbol of slot 0 and slot 10. In other words, the P-SCH and S-SCH of an FDD system frame occupy contiguous OFDM symbols. As shown by FIG. 5, in a TDD radio 15 frame, the Primary Synchronous channel (P-SCH) is located in 72 sub-carriers in the middle of the third OFDM symbol in the DwPTS domain of sub-frame 1 and sub-frame 6, and the Secondary Synchronous channel (S-SCH) is located in 72 sub-carriers in the middle of the last OFDM symbol of slot 0 and slot 5. In other words, P-SCH and S-SCH of a TDD system frame are 3 OFDM symbols apart.
In an LTE system, the transmission period of the Primary Broadcasting Channel (P-BCH) is 40 ms. The period is split into 4 P-BCH bursts that are respectively mapped to slot 1 of 4 radio frames within the period. As shown by FIG. 4 and FIG. 5, in both the FDD system and the TDD system, the P-BCH burst is mapped to the first 4 OFDM symbols of slot 1 in time, and occupies 72 sub-carriers in the middle of bandwidth in the frequency domain.
In that case, in a LTE system, the User Equipment (UE) detects a service cell by the following process: first, P-SCH is synchronized with S-SCH to detect Physical Identity of Cell (PID), because the relative positions of P-SCH and S-SCH in the FDD system and TDD system frames are different, an FDD system can be distinguished from a TDD system based on the relative positions of the P-SCH and the S-SCH. Second, the UE detects the position of CRS so as to verify whether the detected PID is an effective cell. Last, the UE demodulates the P-BCH so as to obtain primary broadcasting information of the cell.
Reducing the costs of the subsequent compatible control signaling and CRS, and reducing the disturbances introduced by the subsequent compatible control signaling and CRS, will improve the utility rate of the frequency spectrum of the UE in further evolutions of the LTE system. The reduction in the cost of CRS also improves the power saving performance of the system. PDCCH and PDSCH transmissions in such a system are generally demodulated based on DMRS, which is generally referred to as a New Carrier Type (NCT).
As a result of the demodulation in an NCT cell being based on DMRS, all of the legacy UEs cannot work in an NCT cell, because the legacy UEs receive control information based on CRS to some extent. As for the legacy UE, when it attempts to initially access to a cell that operates in the NCT pattern, such initial access is doomed to fail.
NCT cells can be categorized into two types, in accordance with whether it can operate as a standalone cell. In a non-standalone case, it can only operate as a Secondary Cell (Scell) in a Carrier Aggregation (CA) system, and if the NCT cell cannot synchronize with other CA cells, P-SCH and S-SCH are still required, but P-BCH and other broadcasting information no longer need to be transmitted, resulting in reduced costs. In the standalone case, all channels from the legacy LTE system are in need of corresponding defined alternatives in the NCT system, particularly, P-SCH, S-SCH and P-BCH are required to be transmitted. Therefore, there is a need to provide a method and device for transmitting P-SCH, S-SCH and P-BCH in an NCT system.